Add job cancellation to the stats dashboard

- Active jobs on /stats get a cancel button wired to the existing
  download/match cancel endpoints, showing "cancelling..." and an inline
  error when the task already finished.
- Cancelling now sets the status immediately, so a task whose runner died
  in a restart stops showing as "downloading".
- Download streams use a bounded read timeout (10 s connect / 60 s read):
  a stalled socket fails within a minute (previously it could block
  forever), and a task cancelled while stalled is marked cancelled rather
  than error.
- The stats job list reaps stale download/match tasks, so phantom jobs
  never appear on the dashboard.

backend/apps/core/stats.py

@@ -122,6 +122,7 @@ def _download_job(task):
         "downloaded": task.downloaded,
         "total": task.total,
         "detail": detail,
+        "cancelled": task.cancelled,
         "created_at": task.created_at,
         "updated_at": task.updated_at,
     }
@@ -143,6 +144,7 @@ def _match_job(task):
             f"{task.processed}/{task.total} · {task.matched} matched · "
             f"{task.not_found} not found · {task.deleted} deleted"
         ),
+        "cancelled": task.cancelled,
         "created_at": task.created_at,
         "updated_at": task.updated_at,
     }
@@ -150,8 +152,14 @@ def _match_job(task):
 
 def jobs_stats():
     """Running/pending jobs plus the most recently finished ones."""
+    from apps.library.downloads import reap_stale_downloads
+    from apps.library.matching import reap_stale_match_tasks
     from apps.library.models import DownloadTask, MatchTask
 
+    # Tasks whose runner died in a restart must not linger as "downloading".
+    reap_stale_downloads()
+    reap_stale_match_tasks()
+
     active_statuses_download = [
         DownloadTask.STATUS_PENDING,
         DownloadTask.STATUS_DOWNLOADING,